Present embodiments relate generally to gas turbine engines. More particularly, but not by way of limitation, present embodiments relate to methods of preparing a material with a laser for welding.
In turbine engines, air is pressurized in a compressor and mixed with fuel in a combustor for generating hot combustion gas which flow downstream through turbine stages. These turbine stages extract energy from the combustion gas. A high pressure turbine includes a first stage nozzle and a rotor assembly including a disk and a plurality of turbine blades. The high pressure turbine first receives the hot combustion gas from the combustor and includes a first stage stator nozzle that directs the combustion gas downstream through a row of high pressure turbine rotor blades extending radially outwardly from a first rotor disk. In a two stage turbine, a second stage stator nozzle is positioned downstream of the first stage blades followed in turn by a row of second stage turbine blades extending radially outwardly from a second rotor disk. The stator nozzles direct the hot combustion gas in a manner to maximize extraction at the adjacent downstream turbine blades.
The first and second rotor disks are joined to the compressor by a corresponding rotor shaft for powering the compressor during operation. These are typically referred to as the high pressure turbine. The turbine engine may include a number of stages of static airfoils, commonly referred to as vanes, interspaced in the engine axial direction between rotating airfoils commonly referred to as blades. A multi-stage low pressure turbine follows the two stage high pressure turbine and is typically joined by a second shaft to a fan disposed upstream from the compressor in a typical turbofan aircraft engine configuration for powering an aircraft in flight.
As the combustion gas flows downstream through the turbine stages, energy is extracted therefrom and the pressure of the combustion gas is reduced. The combustion gas is used to power the compressor as well as a turbine output shaft for power and marine use or provide thrust in aviation usage. In this manner, fuel energy is converted to mechanical energy of the rotating shaft to power the compressor and supply compressed air needed to continue the process.
During the operation of the gas turbine engine, it is necessary to obtain temperature readings at different locations in the engine. This data is utilized by the engine control logic to properly operate the engine and provide maximum performance at the highest efficiency. One such temperature probe which is utilized at the exhaust area of the combustor, it is known as an Exhaust Gas Temperature probe or EGT probe. These probes utilize thermocouples, typically having a dissimilar metal to create a differential which may be then related to a temperature which is provided to the engine control logic. The thermocouple wires are disposed in a sheath with an insulating magnesium oxide. To prepare the thermocouple wires for installation and welding, the thermocouple sheath is stripped away from the portion of the thermocouple wire or lead which is to be welded. The magnesium oxide powder must also be cleaned away.
Various methods have been attempted in order to perform this cleaning. The magnesium oxide powder has been cleaned with alcohol in an attempt to remove such from the wire. Additionally, rotating blades have been utilized in an attempt to mechanically remove the magnesium oxide. As a further alternative, abrasive pads have been utilized in an attempt to manually remove the magnesium oxide powder. Unfortunately, the thermocouple leads are extremely costly and mechanical and other means of cleaning have resulted in damage and discarding of an unacceptable percentage of the thermocouples.
Of the remaining thermocouples which are not damaged in the mechanical cleaning process, various of these structures have problems with the weld bond due to remaining contaminants on the surface of the thermocouple lead or wire. Specifically, for example, magnesium oxide powder may not be thoroughly cleaned from the surface and therefore results in welds which have high porosity in the weld, low percentage of weld fusion across the wire diameter and low percentage of weld fusion. The fractures of a poor weld bond are depicted in FIG. 8 across the terminal.
As may be seen from the foregoing, there is a need to optimize the cleaning procedure of materials which will be welded while limiting damage during the process so that the optimal bond may occur when the weld process occurs.